Save composer.json file to same location no matter where wp-cli command was run

Sometimes the composer.json file will be saved to the directory where the wp-cli command was run from. The composer.json file should always be saved to the same place (preferably the site root).

This occurs when using commands such as:
```
wp plugin install akismet
wp theme install twentysixteen
```

A new composer.json file will be created in the directory where the command was run.